Abstract: Electronic cigarettes are rapidly becoming popular as a substitute for cigarette smoking and as an adjunct for smoking cessation. Unfortunately, little is known about use of these new tobacco products in pregnant women. Although the best option would be for pregnant women to stop using tobacco products entirely, a central question for women who do not quit smoking is ?Are electronic cigarettes a net benefit or harm in terms of toxicant exposure and birth outcomes?? Because electronic cigarettes do not burn tobacco, they may reduce carbon monoxide (CO) exposure (a toxin that may contribute to low birth weight and neurotoxicity) and carcinogen exposure, which has been linked to low birth weight and childhood cancer. However, a concern exists that electronic cigarettes may present risks in pregnancy that match or exceed those of cigarette smoking such as increased nicotine exposure (particularly in dual users), and an increase formaldehyde exposure, which has been linked to adverse reproductive outcomes in nonsmokers. We will recruit 125 pregnant smokers and 250 pregnant electronic cigarette users (125 women who use e- cigs exclusively during pregnancy, and 125 dual users). We will advise women to stop all tobacco use, and provide written materials and quitline referrals. Maternal toxicant exposure levels of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), formaldehyde, and cotinine (major metabolite of nicotine) will be obtained throughout pregnancy and infant levels of NNAL and cotinine will be obtained at the time of delivery. We will also obtain data on birth weight, gestational age, and other reproductive outcomes. Our primary objective is to compare toxicant exposure in pregnant women who use electronic cigarettes compared to women who smoke conventional cigarettes. We hypothesize that women who smoke conventional cigarettes exclusively will have higher NNAL and cotinine concentrations throughout pregnancy and at delivery than electronic cigarette users. We will also explore differences in urine formaldehyde levels. Another primary objective is to examine the impact of conventional cigarettes compared to electronic cigarettes on birth weight. We hypothesize that infants born to exclusive conventional cigarette smokers will have lower birth weights than those of electronic cigarette users. We will also examine mechanisms by which conventional and electronic cigarettes could impact birth weight and other reproductive outcomes using toxicant biomarkers as well as biomarkers of inflammation. Results from this project can be used to better inform clinicians who may be in a position to counsel pregnant women about the potential effects of electronic cigarette use during pregnancy.